cmd/go: added missing space to pkg-config errors
cmd/go: added missing space to pkg-config errors

pkg-config errors were missing a space between pkg-config flags and packages
e.g.  pkg-config --cflags --staticsdl2

Fixes #25260

Change-Id: Iacecb697c5782c4822db9eefa1a2e4c4daf91a69
GitHub-Last-Rev: 2adf8576be66e31160c109a79f3853feac293f1b
GitHub-Pull-Request: golang/go#25261
Reviewed-on: https://go-review.googlesource.com/111638
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/go,nawawi/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,josharian/go,CAFxX/go,CAFxX/go,golang/go,akutz/go,josharian/go,akutz/go,josharian/go,josharian/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,josharian/go,nawawi/go,nawawi/go,josharian/go,josharian/go,golang/go,CAFxX/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,akutz/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,akutz/go